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1、HibbelerR.C.“Force-SystemResultantsandEquilibrium”ThermalDesignofElectronicEquipment.Ed.RalphRemsburgBocaRaton:CRCPressLLC,2001©2001byCRCPRESSLLC0082-02Page19Wednesday,August23,20009:53AM2FormulasforConductiveHeatTransfer2.1CONDUCTIONINELECTRONICEQUIPMENT:INTRODUCTIONHeattr
2、ansferbytheconductionmodeoccurswhenheatistransferredwithinamaterial,orfromonematerialtoanother.Theenergytransferispostulatedtooccurbecauseofkineticenergyexchangebyelasticandinelasticcollisionsofatoms,andbyelectrondrift.Heatenergyisalwaystransferredfromaregionofhigherenergyt
3、oanareaoflowerenergy.Theenergylevel,ortemperature,ofamaterialisrelatedtothevibrationlevelofthemoleculeswithinthesubstance.Iftheregionsareatanequaltemperature,noheattransferoccurs.Fourier’slawcanbeusedtopredicttherateofheattransfer.1Thelawsuggeststhattherateofheattransferbep
4、roportionaltotheareaoftransfertimesthetemperaturegradientdT/dx.dTqkA-------dxInFourier’slaw,therelationT(x)isthelocaltemperatureandxisthedistanceofheatflow.Althoughthisisanequationofproportionality,theactualrateofheattransferdependsonthethermalconductivity,k,whichisaphysica
5、lpropertyoftheheattransfermedia.ThermalconductivityisgenerallyexpressedintermsofW/mK.Heattransfercanoccurbyconductionthroughanymaterial:solid,liquid,orgas.Conductioncannotoccurthroughavacuumbecausethereisnomaterialtoconductthrough.Conductionisnotusuallythepredominantmethodo
6、fheattransferthroughagasorliquid.Usually,asweapplyheattoafluid,theheatedportionofthefluidexpandsandsetsupdensitygradients.Thesedensitygradientscausemotionwithinthefluid,whichleadstoconvectiveheattransfer.Convectiveheattransfer,amacroscopicmethodofenergytransfer,ismuchmoreeffec
7、tivethanconductiveheattransfer.Thevaluesusedforthethermalconductivitiesofliquidsandsolidsaregenerallyobtainedbyexperimentation.Thethermalconductivityofgasesatmoderatetem-peraturescloselyfollowsthekinetictheoryofgases,andthereforecalculatedvaluesmaybeused.©2001byCRCPRESSLLC0
8、082-02Page20Wednesday,August23,20009:53AM2.2THERMALCONDUCTIVITYFourier’slawpresent
